>>>>> "Ted" == Theodore Ts'o <tytso@xxxxxxx> writes: >> It's dubious. I'm working on making sure we only set >> discard_zeroes_data when the device guarantees it for 3.19. Ted> I'm curious how you were going to determine that the device Ted> guarantees that discard_zeroes_data will be honored. Is there some Ted> new bit in some mode page that promises that discards won't be Ted> thrown away any time the device feels like it? We discussed this a week or two ago over on linux-raid because RAID5 depends on hard guarantees in the discard_zeroes_data department. SCSI UNMAP suffers from the same lame behavior as ATA TRIM in the sense that a device can report that it supports zero after UNMAP. But there is no guarantee that all parts of an UNMAP command will be processed by the storage device. Only parts that were actually processed will return zeroes on subsequent reads. And obviously we don't know which parts the device decided to ignore. *sigh* It's incredibly frustrating that it is so hard to get the standards bodies to give any guarantees about anything. It's an absolute miracle that a READ after a WRITE is required to return the same data. Anyway. Contrary to UNMAP, WRITE SAME with the UNMAP bit set requires subsequent reads to deterministically return zeroes. If a block can't be unmapped for whatever reason it will be explicitly zeroed. So I'm working on a patch set that will: - Set discard_zeroes_data for SCSI devices that support the WRITE SAME w/ UNMAP commands. - Not set discard_zeroes_data for SCSI devices that only support UNMAP. - Set discard_zeroes_data for certain ATA SSDs that are known to behave correctly. It's a royal pain to have to maintain a whitelist. But all the hardware RAID vendors do the same thing. I'm talking to various folks to leverage any guarantees we and other vendors may have in existing product requirement documents. As you alluded to, there are devices that otherwise appear to be entirely well-behaved that can get stressed and start dropping discards. That may be acceptable for the ext4 use case but will cause corruption for RAID5. So we are compelled to being more conservative about setting discard_zeroes_data. Until my tweaks are in Neil has opted to disable discard on RAID5. Martin (I scream daily) -- Martin K. Petersen Oracle Linux Engineering -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
